This invention relates to the assembling and packaging of integrated circuit devices and, more particularly, to a lead frame for such devices, a method for trimming and forming an encapsulated device, and to an improved encapsulated device.
It is known to encapsulate integrated circuits with plastic. See, for example, U.S. Pat. No. 4,043,027, the disclosure of which is here incorporated by reference. Integrated circuits in the form of semiconductor dies are first attached to a support pad of the lead frame. Contact or bond pads on the semiconductor device are then individually attached by wire bonding to corresponding contact pads on the ends of the leads.
After the wire bonding operation is completed, the lead frame is placed in a mold. The mold is provided with a reservoir for containing a quantity of insulating, molding compound. The molding compound is injected into the mold in order to encapsulate the integrated circuit. Such injection molding is accomplished as mentioned in the aforesaid patent and is further described in U.S. Pat. No. 4,504,435, also incorporated by reference.
It is found useful by those skilled in the art to form the lead frames in a continuous strip. Each lead frame of a strip has an integrated circuit device attached to the support pad as mentioned above. The support pads are themselves supported by a pair of support arms between two parallel siderails. Each siderail is located in the plane of the lead frame and on opposite sides of the die pad. In the popular, so-called dual in-line package, one lead frame serially follows the next. In another so-called matrix package, a plurality of lead frames are disposed between opposing siderails with transverse carrier arms disposed between the two siderails for receiving the support arms of the die pad.
In the molding operation, mold cavities are formed around the lead frames to tightly close and seal upon the leads themselves as well as the dam bar. The dam bar has a transverse portion that extends between a pair of adjoining leads. The dam bar restricts the flow of encapsulated material from the enclosed lead frame. After encapsulation, the dam bar and a portion of the mold flashing projecting between adjoining leads is removed by a punch. The punch is the typical metal punch that readily severs the metallic dam bar and also removes a portion of the projected mold flashing from between the lead frames.
The removal of the mold flashing by the dam bar punch presents several problems. The molding compound itself includes abrasive material such as silicon dioxide. This abrasive material wears away the cutting edge of the dies thereby reducing their useful life. Moreover, as the dies wear out, the flashing is unevenly and jaggedly removed. As such, the portion of the flashing remaining between the leads has a jagged edge. Such a jagged edge mold flashing causes further problems when automatic machinery is used to insert the encapsulated integrated circuit into tightly packed printed circuit boards. The extraneous, jagged mold flashing interferes with such automatic insertion machinery thereby decreasing the efficiency and speed of the final assembly of an electronic device.
As such, there has arisen a need for an improved lead frame for producing an encapsulated integrated circuit that has outside edges that are well defined by the molding process itself without the need for mold flash removal by a metal cutting die. There has also developed a further need for reducing the extent of the mold flashing so that such flash is within acceptable tolerance limits of automatic insertion machines. There is also a need for improving the trim and forming operations in the manufacture of an integrated circuit to increase the longevity of the dies that are used to sever the lead frame from its siderails.